It depends on the chemical. Some react to sunlight and quickly degrade, like atrazine. It is one that is safe to use frequently (following label directions, of course).

Others can be quite persistent and will go back into solution in a heavy dew. Rain will carry it into streams. Every major river in the USA has herbicides in it, mostly from forestry and agricultural application. By draining backwater swamps and oxbows, we have decreased the ability of rivers to self-clean. The result is that the farther downstream we go, the more varied the chemical soup we produce.

Any household that receives water from a major river is receiving chlorinated or fluoridated agricultural chemicals. The concentrations of these chemicals is so low that nobody has ever been able to scientifically detect a causal link between the water and health effects.

Something like 75% of the population of the USA lives within 100 miles of the coast. For the most part, major coastal cities are situated on major rivers and draw their potable water from those rivers.

B